1. Field of the Invention
The invention relates to a thin-film device, and a method of fabricating the same.
2. Description of the Related Art
A transparent electrically conductive film composed of oxide, such as an ITO film composed of compound of indium (In), tin (Sn), and oxygen (O), is frequently used in a flat panel display or a photoelectric transfer device, since it has a sheet resistance of a few ohms per a unit area even if it has a small thickness such as hundreds of nanometers, and it has high transmittance to visible light.
Furthermore, a study to a thin-film transistor including a channel layer composed of transparent oxide semiconductor such as In—Ga—Zn—O has been recently started. Such oxide semiconductor contains highly ionic bonds, and is characterized by a small difference in electron mobility between crystalline state and amorphous state. Accordingly, relatively high electron mobility can be obtained even in amorphous state.
Since an amorphous film of oxide semiconductor can be formed at room temperature by carrying out sputtering, a study about a thin-film transistor composed of oxide semiconductor to be formed on a resin substrate such as a PET substrate has been started.
For instance, a thin-film transistor composed of oxide semiconductor is suggested in Japanese Patent Application Publication No. 2005-033172 (paragraph 0041), Japanese Patent Application Publication No. 2003-179233 (paragraphs 0014-0016), Japanese Patent Application Publication No. 2003-86808 (paragraph 0053), Japanese Patent Application Publication No. 2003-60170 (paragraph 0037), and Japanese Patent Application Publication No. 2006-502597 (paragraphs 0021-0023).
In a thin-film transistor composed of oxide semiconductor, donor defects caused by oxygen holes existing in a semiconductor film, in particular, oxygen holes existing at an interface layer between a semiconductor film and an electrically insulating film exert much influence on electric characteristics of the thin-film transistor.
The above listed Publications are accompanied with a problem of insufficient control to oxygen holes existing at an interface layer.
In particular, Japanese Patent Application Publication No. 2006-502597 alleges that it is possible to reduce oxygen holes by annealing oxide semiconductor at 300 degrees centigrade or higher in oxidation atmosphere. Though such annealing may be effective to reduction in oxygen holes existing at an upper surface of an oxide-semiconductor film, such annealing is not effect to reduction in oxygen holes existing at a lower surface of an oxide-semiconductor film (that is, a region of oxide semiconductor close to an interface between an underlying insulating film and oxide semiconductor formed on the underlying insulating film). This is because such annealing is difficult to penetrate an oxide-semiconductor film.
It may be possible to oxidize even a lower surface of an oxide-semiconductor film by carrying out annealing at 600 degrees centigrade or higher for enhancing penetration of oxidation, however, in which case, there would be caused problems that it is not possible to use a cheap glass substrate as an electrically insulating substrate, and that if a metal film exists below an oxide-semiconductor film, annealing causes metal of which the metal film is composed to diffuse into the oxide-semiconductor film, and resultingly, the oxide-semiconductor film is contaminated.
Thus, it was not possible to fabricate a thin-film transistor composed of oxide semiconductor and having desired characteristics sufficiently applicable to a display driver, on a cheap glass substrate with high reproducibility and high fabrication yield.